Increasingly common spinal surgical procedures, such as those performed to treat deforming spinal conditions for example, often require immobilizing or fusing vertebrae together by installing surgical implants to the affected bone structures. The implantation of pedicle screw fixation systems to correct and stabilize congenital and acquired deformities, trauma and degenerative diseases of the spine is well known. Such implants often include, for example, various screws or hooks fixed to vertebrae and linked by rods, plates or wires. Many different vertebra mounted spinal implants are known, however most share a common requirement for fixation of an element to a vertebral body: screw insertion. The use of screws, installed into the pedicle region of each vertebra requiring immobilization, remains a critical mainstay of spinal corrective surgery, but one fraught with a potential for error that, even given today's surgical devices and procedures, remains significant.
Traditional spinal surgical operations are highly invasive, often requiring large incisions which, while necessary to achieve sufficient spinal exposure, result in significant patient trauma and post operative pain. Computer assisted image guided surgical instrument navigation and percutaneous implant fixation are increasingly being used wherever possible in an effort to reduce the invasiveness of spinal surgery.
It is well known to use vertebral clamps having opposing jaws with teeth thereon for driving together and penetrating tissue and bone such that the clamp can be fixedly fastened to a spinous process of a vertebral body. Such clamps are typically quite long, about 20 cm in length, although smaller clamps intended to significantly limit the invasiveness of spine surgery, have more recently also emerged. The use of these clamps with an image guided surgery system, wherein the clamp comprises a trackable element thereon which can identify the position and orientation of the clamp, and therefore also that of the bone element to which it is fixed, is also well known.
Computer Assisted Surgery (CAS) systems have considerably enabled much more precise and accurately repeatable pedicle screw insertion during spinal surgical operations. However, surgical navigation in the spine, where multiple vertebral bodies can move relative to each other, as a result of patient breathing or other body movement for example, creates navigation difficulties unique to spinal surgery in general, and to pedicle screw placement and insertion in particular. It has been recently taught, for example by Bucholz et al. in U.S. Pat. No. 6,236,875, to modify and reposition the pre-operatively acquired anatomical image data set to match the actual position of the anatomical elements during the surgery. A single tracked clamp is fixed to a first vertebra into which a pedicle screw is to be placed, for example. Once the vertebra is registered with the computer model of itself, the trackable clamp fastened thereto permits tracking this registered vertebra only using the CAS system. Once work on the registered vertebra is completed, the entire clamp assembly is removed, and reinstalled on a second vertebra, which once registered, can then be tracked by the system. Therefore, in order to insert pedicle screws into a plurality of vertebrae, these steps must be repeated for each vertebra sequentially, and therefore only one vertebra is ever accurately tracked and therefore precisely located in space by the CAS system, at any one time. While only limited displacement between vertebrae naturally occurs, even the slightest movement of the patient during surgery, for example movement resulting from breathing, can cause enough relative displacement of the other vertebrae with respect to the one being tracked to cause their inaccurate depiction. Therefore the precise position and movement of the entire section of interest of the spine cannot be accurately located with only a single vertebra being tracked. Using individual tracking elements for each rigid body in spine surgery, while permitting intra-operative detection of each anatomical element, is impractical as having detectable members fixed to each vertebra takes up considerable space within the surgical field, limiting the workspace available to the surgeon, and often additionally resulting in a higher level of invasiveness. This is, however, what has been done to date.
In U.S. Pat. No. 6,226,548 Foley et al. disclose a CAS navigated apparatus which provides the ability to track a surgical instrument and implants such that percutaneous surgical operation can be performed on anatomical elements, such as vertebrae, which can intra-operatively move relative to each other. Foley et al. use a percutaneous bone clamp, which is fastened to a spinous process of a vertebra, and onto which is removably engaged a superstructure having a reference member including emitters for providing a positive indication of movement of the structure, and hence the vertebra to which it is fastened, to the surgical navigation system. The system can thereby positively indicate to the surgeon a change in position of the vertebrae from that identified in the pre-stored image scan, and provides a method to realign those body parts to correspond with a previously stored image or the image to correspond with the actual current position of the body parts.
However, if each rigid body is not individually detectable, any movement of the anatomical structures relative to one another during surgery can cause the pre-operatively scanned images to misrepresent the intra-operative anatomy, a possibility that could eventuate in very grave consequences. When a single vertebra is being tracked, only the spatial position and orientation of the lone tracked vertebral body can be accurately determined.
Additionally, when the spinal column in particular is the multiple-bone anatomical structure of interest, there exists difficulty in tracking all of its constituents. While spinal column bone clamps adapted to be fastened onto vertebra are known, whether tracked or not, it generally remains difficult to track the sacrum with these standard vertebral clamps currently in use. As the sacrum generally has very little process portion which protrudes, the sacrum provides no convenient clamping point permitting simple engagement of a tracked bone reference clamp. Without using a bone reference which requires fastening to the sacrum body using bone screws for example, it is currently difficult to use standard vertebrae reference clamps in order to identify and track the sacrum with an image guided surgery system.
Therefore, there nevertheless remains a need to provide a simple method of tracking a multi-bone moveable structure, such as the spinal column, and registering it with an anatomical image or model, such that the location of the multiple-bone moveable structure can be accurately determined intra-operatively by an image guided surgical system, without requiring displacement of the bone reference apparatus from one bone to another. There also remains a need to provide a vertebral clamp capable of conveniently engaging the sacrum, in order to permit tracking of the sacrum.